This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 0101902-5 filed in Sweden on May 30, 2001; the entire content of which is hereby incorporated by reference.
The present invention relates to a coated cutting tool for chipforming machining. The coating includes at least one alumina (Al2O3) layer characterized by fine, equiaxed grains.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Cemented carbide cutting tools coated with various types of Al2O3 layers using Chemical Vapour Deposition (CVD), e.g., pure xcexa-Al2O3, mixtures of xcexa- and xcex1-Al2O3 coarse grained xcex1-Al2O3 and finegrained textured xcex1-Al2O3 have been commercially available for years generally in multilayer combinations with other metal carbide and/or nitride layers, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table.
Al2O3 crystallises in several different phases: xcex1, xcexa, xcex3, xcex4, xcex8 etc. The two most frequently occurring phases of CVD-produced wear resistant layers are the thermodynamically stable, xcex1-phase and the metastable xcexa-phase or a mixture thereof. Generally, the xcexa-phase exhibits a grainsize in the range 0.5-3.0 xcexcm (dependent on the layer thickness) and the grains predominately grow through the whole coating forming a columnar type coating morphology.
Furthermore, the xcexa-Al2O3 layers are free from crystallographic defects and also free from micropores and voids. Coarse grained (3-6 xcexcm) xcex1-Al2O3 often possesses porosity and crystallographic defects, while finegrained textured xcex1-Al2O3 are free of defects with very pronounced columnar-shaped grains.
In U.S. Pat. No. 5,674,564 a method is disclosed of growing a fine-grained xcexa-Al2O3 layer by employing a low deposition temperature and a high concentration of a sulphur compound.
In U.S. Pat. No. 5,487,625 a method is disclosed for obtaining a finegrained, (012)-textured xcex1-Al2O3 layer consisting of columnar grains with a small cross section (about 1 xcexcm).
In U.S. Pat. No. 5,766,782 a method is disclosed for obtaining a columnar fine-grained (104)-textured xcex1-Al2O3 layer.
The lifetime and the performance of a coated cutting tool are closely related to the microstructure of the coating materials used. Although, coatings produced according to above-mentioned prior art patents show good cutting properties there is still a strong desire to further improve coating microstructures to suit specific cutting conditions and workpiece materials.
As has been mentioned above, all Al2O3 layers produced by the CVD technique possess a more or less columnarlike grainstructure. An Al2O3 layer with an equiaxed grainstructure is, however, expected to show some favourable mechanical properties, e.g., resistance to crack propagation and higher cutting edge toughness, as compared to a layer with a columnar grainstructure. In addition, finegrained layers generally have smoother surfaces than coarse grained layers. During cutting less workpiece materials will adhere onto a smooth coating surface which in turn will imply lower cutting forces and less tendency for the coating to flake off. Nowadays coated cutting inserts are often brushed with SiC based brushes or blasted with finegrained Al2O3 powder in order to obtain a smooth coating surfaces, a rather costly production step.
One well-known and possible technique to produce a finegrained structure and to restrain a columnar grain growth is to deposit a so-called multilayer structure in which the columnar growth of, e.g., Al2O3, is periodically interrupted by the growth of a 0.05-1 xcexcm thick layer of a different material such as disclosed in U.S. Pat. No. 4,984,940 and U.S. Pat. No. 5,700,569. The latter layer should preferably have a different crystal structure or at least different lattice spacings in order to be able to initiate renucleation of the original layer. One example of such a technique is when the Al2O3 growth periodically is interrupted by a short TiN deposition process resulting in a (Al2O3+TiN)xn multilayer structure with a thickness of each individual TiN layer in the range 0.1-1 xcexcm, e.g., see Proceedings of the 12th European CVD Conference page pr. 8-349. However such multilayer structures very often suffer from a low adherence between the two different types of layers.
In Swedish patent application SE 0004272-1 a method is disclosed for obtaining a finegrained xcex1-Al2O3 layer consisting of essentially equiaxed grains with a grain size  less than 1 xcexcm. The grain refinement is accomplished by periodically interrupting the Al2O3 process and treating the Al2O3 surface with a mixture of TiCl4/H2. When the Al2O3 process is restarted renucleation of the xcex1-Al2O3 will take place.
It is not possible to use this method to produce finegrained xcexa-Al2O3 since only the xcex1-Al2O3 phase will nucleate on the TiCl4/H2 treated Al2O3 surface.
xcexa-Al2O3 and xcex1-Al2O3 coatings used as tool materials have slightly different wear properties when cutting different materials. It is therefore also desirable to have means to produce finegrained xcexa-Al2O3 with a controllable grain structure.
It is the object of the present invention to provide onto a hard substrate, or preferably onto a hard substrate coated with a TiCxNyOz and/or ZrCxNyOz-layer (x+y+z=1 and x,y,zxe2x89xa70) at least one finegrained single phase xcexa-Al2O3 layer with a microstructure which is different from the prior art columnar xcexa-Al2O3 CVD layers mentioned above.
It is also the object of the present invention to provide a high performance tool coating comprising the invented Al2O3 layer.
It is a further object of the invention to provide an alumina coated cutting tool insert with improved cutting performance in steel and nodular cast iron.
According to one aspect of the present invention there is provided a cutting tool comprising a body of sintered cemented carbide, cermet or ceramic, and a hard and wear resistant coating applied on at least a functioning portion of a surface of the body. The coating comprises a structure of one or more refractory layers of which at least one layer consists essentially of an equiaxed finegrained xcexa-Al2O3 with a thickness of 0.5-25 xcexcm and with a grain size of less than 0.5 xcexcm, and said finegrained xcexa-Al2O3 layer comprises at least one sub-layer with a thickness between 0.02 and 3 xcexcm containing Al, Si and O with a Si concentration between 4-34 at %, Al concentration of 0-37 at % and O concentration of 60-67 at %.
According to another aspect, the present invention provides a method of coating a body with at least one finegrained xcexa-Al2O3 layer comprising contacting the body with a reaction mixture comprising a hydrogen carrier gas, one or more halides of aluminium and a hydrolysing and/or oxidising agent at 800-1050xc2x0 C., adding a sulphur agent to the reaction mixture to enhance the growth rate, and depositing at least one silicon enriched sublayer in the xcexa-Al2O3 layer by periodically introducing a silicon halide.